Snail hosts for schistosomes have genes that strongly influence trematode parasite development. Adult resistance in Biomphalaria glabrata for the trematode Schistosoma mansoni follows Mendelian inheritance, making it feasible to study the resistance/susceptible phenotypes by standard molecular methods. The integrated approach of this proposal involves genetic studies of the snail and parasite that influence total cercarial production ( as a measure of snail/ parasite compatibility), biochemical and molecular events influencing diapause behavior in snails, and definition of the locus in the snail genome that codes for resistance to the parasite. Several methods are chosen for studying these issues. These include well-developed procedures for genetic breeding of snails to obtain populations with known and stable phenotypes for susceptibility. Biochemical studies of snails that can estivate successfully will use standard protein and mRNA analysis. RFLP and related mapping techniques will be used for defining the gene locus for resistance to S. mansoni. These targeted issues are important for designing new strategies in reducing the transmission of schistosomiasis. By understanding the genes (and gene products) influencing parasite development or destruction in the snail host, we ultimately may develop methods for the biological control of this disease. Compared to our molecular knowledge of genes controlling resistance for malaria in mosquito vectors, this research direction for molluscs is in its relative infancy. By defining the genes influencing resistance in B. glabrata, it would advance considerably our understanding of the physiological basis for parasite destruction in the intermediate host.